1. Field of the Invention
This invention relates to adhesives used for bonding flow field plates together for use in fuel cells. In particular, it relates to epoxy methacrylate based adhesives for preparing bipolar plate assemblies for solid polymer electrolyte fuel cells.
2. Description of the Related Art
Fuel cells such as proton exchange membrane or solid polymer electrolyte fuel cells (SPEFCs) electrochemically convert fuel (such as hydrogen) and oxidant (such as oxygen or air) to generate electric power. SPEFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
Bipolar plate assemblies comprising an anode flow field plate and a cathode flow field plate which have been bonded and appropriately sealed together so as to form a sealed coolant flow field between the plates are thus commonly employed in the art. Various transition channels, ports, ducts, and other features involving all three operating fluids (i.e. fuel, oxidant, and coolant) may also appear on the inactive side of these plates. The operating fluids may be provided under significant pressure and thus all the features in the plates have to be sealed appropriately to prevent leaks between the fluids and to the external environment. A further requirement for bipolar plate assemblies is that there is a satisfactory electrical connection between the two plates. This is because the substantial current generated by the fuel cell stack must pass between the two plates. Numerous variants of such bipolar plate assemblies appear in the art, for instance as disclosed in US20080107952.
The plates making up the assembly may optionally be metallic, in which case they are typically welded together so as to appropriately seal all the fluid passages from each other and from the external environment. Additional welds may be provided to enhance the ability of the assembly to carry electrical current, particularly opposite the active areas of the plates. Metallic plates may however be bonded and sealed together using adhesives. US20050031933 for instance discloses an elastomeric adhesive (with glass transition temperature below −20° C.) primarily for bonding metallic plates together. A conductive primer layer is required.
The plates making up the assembly may also optionally be carbonaceous (e.g. formed graphite plates) and such plates are frequently sealed together using elastomeric contact seals with the entire stack being held under a compression load applied by some suitable mechanical means. More recently, bipolar plate assemblies are being prepared using adhesives that are capable of withstanding the challenging fuel cell environment. Various resin adhesives have been contemplated for this purpose. Such adhesives are applied by screen printing or are otherwise dispensed in a pattern suitable for isolating each desired fluid cavity. Typically such adhesives must undergo a heat curing step.
Finding suitable adhesives that meet all the requirements for this application can be challenging however. Aside from having sufficient bond strength for mechanical purposes, a low electrical resistance is required as substantial current must flow efficiently through the bonded flow field plates. Further, a suitable adhesive generally should have a high glass transition temperature, Tg, such that it does not behave as an elastomer and thus maintains desirable characteristics over a range of fuel cell operating temperatures. A less obvious requirement perhaps is the need to achieve reproducible, uniform bond gaps since the dimensions and hence tolerances of component thicknesses in a fuel cell stack are typically very small. Further still, for manufacturing purposes, certain viscosity characteristics (e.g. viscosity versus shear as a function of temperature and time) are needed for preferred methods for application (e.g. roller coating) and also for subsequent curing. For instance, the material properties of the adhesive can radically change between the time of application and the time of curing with undesirable consequences (e.g. the viscosity may significantly reduce as the adhesive is heated and can run out of the joint or spread via capillary action across the parts and onto fixtures used to locate and press the plates during curing).
In U.S. Pat. No. 6,933,333, an adhesive formulation is disclosed for purposes of bonding bipolar plate assemblies together for use in solid polymer electrolyte fuel cells. The formulations are based on either vinyl ester or polyester resins. Milled carbon fibres are employed in these formulations in amounts of from about 2 to 20% by weight and provide structural integrity in the cured composite interface. The formulations also include other non-fibrous graphite powder.
Despite the advances made to date, there remains a need for adhesives having better properties for use in bonding bipolar plate assemblies together for fuel cells. This invention fulfills these needs and provides further related advantages.